It is, of course, well known that blood flows to the uterus through a network of arteries and drains through a network of veins. The heart pumps blood through the arteries under considerable pressure (80-180 mm Hg) and these vessels have a thick muscular wall to withstand these forces. The veins are a low pressure system (approximately 13 mm Hg) and the walls of these vessels are comparatively quite thin. Almost all blood flow problems of the uterus are now thought to involve the high pressure arterial system, not the venous system.
Adequate uterine blood flow is so critical to the organism that its arteries are organized in a special highly protective configuration reserved for the certain vital organs, e.g. the brain. It is this protective arrangement that is responsible to a considerable degree for the difficulties associated with measuring uterine blood flow. Specifically, the configuration employed to protect the body's vital organs comprises an arterial loop which is fed by two arteries.
In the case of the uterus, its principal blood supply comes from the left and right uterine arteries which arise from large arteries in the left and right lateral pelvis and curve across the pelvis to meet the uterus at mid cervix. At the cervix each uterine artery bends upward and runs along the lateral aspect of the body of the uterus. Smaller arteries (arterioles) branch from this vessel and extend into the body of the uterus. By the best current estimates, the left and right uterine arteries supply about 80-90% of the blood supply of the uterus. A protective loop is formed by the small ovarian artery which arises from the aorta and joins the uterine artery near the top of the uterus. This dual arterial supply can provide enough blood to keep the uterine tissue alive if a uterine artery becomes blocked.
A very useful diagnostic tool arises from the fact that blood flow in the uterine and ovarian arteries varies with pressure and, therefore, is phasic in nature. Actually, the pattern is a bi-phasic one in that as the heart contracts and forces a bolus of blood out into the arteries, there is a sharp increase in flow in the uterine arteries. Conversely, during the cardiac relaxation phase, uterine artery flow shows a variable pattern of decline. The flow patterns of the high velocity (systolic) phase and the low velocity (diastolic) phase provide a great deal of information about the physiologic state of the uterus, and, more importantly, a fetus, which would be of considerable value if, in fact, it could be measured.
Accordingly, researchers and physicians have long sought a non-invasive method to measure blood flow in the uterine arteries of human subjects. Normal function of the reproductive system in a human depends upon adequate blood flow to the reproductive organs. It is critical to growth and to the normal physiologic activity of the pregnant and non-pregnant uterus. Impaired blood flow during pregnancy is associated with retardation of fetal development and brain damage, a major cause of perinatal mortality and permanent neurologic disability. Abnormalities of blood flow may also be implicated in several pathologic conditions associated with the uterus such as, for example, dysmenorrhea, chronic pelvic pain syndromes, and possibly endometriosis and other conditions of the non-pregnant state. The dynamics of uterine blood flow in humans are not well understood because a practical method of measuring uterine blood flow has yet to be achieved. As a consequence, most diagnoses of abnormal uterine artery blood flow are presumptive, i.e. inferred from pathologic findings consistent with impairment of the blood supply.